Reinforcements of commercially available or experimental dental composites include glass and silica particles (spherical or irregular in shape), glass-ceramic or oxide ceramic particles, ceramic whiskers, electrospun polymer or hybrid fibers, organic-inorganic hybrid particles, glass fibers, clay, and polyhedral oligomeric silsesquioxanes (POSS). Improvements in properties have occurred to varying extents after incorporation of sophisticated filler blends, but further enhancements aimed at extending the service life and utilization of dental restorative composites are desired. The current paradigm for designing composites is that structure matters more than composition when determining properties. It is therefore surprising that there have been few (if any) attempts to design dental composites containing an ordered or periodic arrangement of the reinforcing phase. Our innovation is to utilize the self-assembly capability of monodisperse, nanoscale colloidal silica particles to fabricate composites with an ordered and close-packed filler structure. Two methods of producing composites will be implemented: a wet colloidal crystal process, and a synthetic opal process. We propose two specific aims to be achieved over three years: 1.) to determine the influence of particle size, particle surface treatment, dispersion medium, and matrix resin on the efficiency of colloidal particle redispersion and ordering;and 2.) to determine the effects of filler particle ordering on mechanical properties including modulus, failure strength, and toughness. Our experimental design calls for evaluating three particle sizes, four silane coupling agents, seven matrix resins, four levels of initiator, and three filler volume fractions. We will test these hypotheses: 1.) that an ordered filler arrangement activates new deformation modes in composites;and 2.) that the strength of composites depends on the degree of filler ordering. The proposed research will lead to the development of novel dental composites with improved esthetics and lower polymerization shrinkage (and reduced incidence of recurrent decay). There is also evidence that the ordered filler arrangement leads to greater reinforcing efficiency and higher toughness. These factors will expand the indications for low-cost composite restorations.